U.S. patent application number 10/761524 was filed with the patent office on 2004-08-26 for vessel sealing system.
Invention is credited to Buysse, Steven P., Orszulak, James H., Wham, Robert.
Application Number | 20040167508 10/761524 |
Document ID | / |
Family ID | 32867448 |
Filed Date | 2004-08-26 |
United States Patent
Application |
20040167508 |
Kind Code |
A1 |
Wham, Robert ; et
al. |
August 26, 2004 |
Vessel sealing system
Abstract
A method for electrosurgically sealing a tissue includes steps
of: (A) applying a first pulse of RF energy to the tissue; and (B)
applying at least one subsequent RF energy pulse to the tissue and
varying RF energy parameters of individual pulses of subsequent RF
energy pulses in accordance with at least one characteristic of an
electrical transient that occurs during the individual pulses of
the subsequent RF energy pulses. The method terminates the
generation of subsequent RF pulses upon a determination that the
electrical transient has passed a predetermined limit.
Inventors: |
Wham, Robert; (Boulder,
CO) ; Buysse, Steven P.; (Longmont, CO) ;
Orszulak, James H.; (Nederland, CO) |
Correspondence
Address: |
United States Surgical
a Division of Tyco Healthcare
150 Glover Avenue
Norwalk
CT
06856
US
|
Family ID: |
32867448 |
Appl. No.: |
10/761524 |
Filed: |
January 21, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10761524 |
Jan 21, 2004 |
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10073761 |
Feb 11, 2002 |
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Current U.S.
Class: |
606/32 ; 606/40;
606/49 |
Current CPC
Class: |
A61B 2018/00827
20130101; A61B 2018/00601 20130101; A61B 18/1445 20130101; A61B
2018/00702 20130101; A61B 18/1206 20130101; A61B 2018/00726
20130101; A61B 2018/00684 20130101; A61B 17/12 20130101; A61B
2018/00404 20130101; A61B 18/1442 20130101; A61B 2018/0063
20130101; A61B 2018/00875 20130101; A61B 2018/00791 20130101 |
Class at
Publication: |
606/032 ;
606/040; 606/049 |
International
Class: |
A61B 018/18 |
Claims
What is claimed is:
1. A method for electrosurgically sealing tissue, comprising the
steps of: applying a first pulse of RF energy to the tissue; and
applying at least one subsequent RF energy pulse to the tissue and
varying RF energy parameters of individual pulses of subsequent RF
energy pulses in accordance with at least one characteristic of an
electrical transient that occurs during the individual pulses of
the subsequent RF energy pulses.
2. A method as in claim 1, wherein the step of applying the first
pulse includes a step of selecting characteristics of the first
pulse so as not to appreciably heat the tissue.
3. A method as in claim 1, wherein the step of applying the first
pulse comprises the steps of: continuously measuring at least one
characteristic of a response of the tissue to the applied first
pulse; and in accordance with the measured characteristic,
determining whether to change a set of RF energy parameters to a
default set of RF energy parameters.
4. A method as in claim 3, wherein the default set of RF energy
parameters comprise a magnitude of a starting power and a magnitude
of a starting voltage.
5. A method as in claim 1, wherein the electrical transient is an
electric current transient.
6. A method as in claim 5, wherein the at least one characteristic
of said electrical transient is a rate of change of the electric
current transient.
7. A method as in claim 1, wherein the electrical transient is
tissue impedance and the at least one characteristic of said
electrical transient is a rate of change of the tissue
impedance.
8. A method as in claim 1, wherein the RF energy parameters that
are varied for individual pulses of the subsequent RF energy pulses
comprise RF power output, current and voltage.
9. A method as in claim 1, further comprising the step of
determining if the tissue responded to the first pulse of RF energy
prior to the step of applying at least one subsequent RF energy
pulse.
10. A method as in claim 9, wherein the step of applying at least
one subsequent RF energy pulse includes the step of varying at
least one of RF starting power, a magnitude of starting current,
and a magnitude of starting voltage for the at least one subsequent
RF energy pulse.
11. A method as in claim 1, further comprising the steps of:
measuring the at least one characteristic of the electrical
transient that occurs at the end of one of the first pulse and the
at least one subsequent RF energy pulse; in accordance with the
measured characteristic, determining whether to terminate the
method for electrosurgically sealing tissue, or using the measured
characteristic to determine a set of RF energy parameters for a
subsequent RF energy pulse and repeating the applying step.
12. A method as in claim 11, wherein the set of RF energy
parameters for the subsequent RF energy pulse comprise a magnitude
of a starting RF power, a magnitude of a starting current, a
magnitude of a starting voltage, and a duty cycle.
13. A method as in claim 11, wherein the electrical transient is an
electrical impedance of the tissue.
14. A method as in claim 13, wherein the step of using the measured
characteristic to determine the set of RF energy parameters for the
subsequent RF energy pulse comprises a step of using the measured
impedance value to readout the set of RF energy parameters from an
entry in one of a plurality of lookup tables.
15. A method as in claim 10, wherein said one of the plurality of
lookup tables is selected manually or automatically, based on a
choice of an electrosurgical tool or instrument.
16. A method as in claim 3, further comprising the step of
modifying predetermined pulses of the set of RF energy parameters
in accordance with a control input from an operator.
17. A method as in claim 16, wherein the predetermined pulses of
the set of RF energy parameters that are modified comprise a pulse
power and a pulse starting voltage.
18. A method as in claim 1, further comprising the step of
terminating a generation of subsequent RF energy pulses upon a
determination that the electrical transient is absent.
19. A system for electrosurgically sealing tissue, comprising an
electrosurgical generator comprising an RF energy source and a
controller for controlling the operation of an electrosurgical
generator, said electrosurgical generator having an output for
coupling to a surgical instrument comprising electrodes for
coupling RF energy generated by said electrosurgical generator to
tissue to be sealed; said controller being operable for causing
said electrosurgical generator to apply an initial pulse of RF
energy to the tissue and for measuring a value of an electrical
characteristic of the tissue in response to the applied initial
pulse, said controller being responsive to the measured electrical
characteristic for determining an initial set of pulse parameters
for at least one subsequent pulse and for then varying the pulse
parameters of individual pulses of further subsequent RF energy
pulses in accordance with a change in the electrical characteristic
of the tissue as determined from at least one characteristic of an
electrical transient that occurs during each individual pulse of
the subsequent RF energy pulses.
20. A system as in claim 19, wherein the electrical characteristic
is comprised of an electrical impedance.
21. A system as in claim 19, wherein the at least one
characteristic of the electrical transient is the rate of change of
the electrical transient.
22. A system as in claim 19, wherein said initial set of pulse
parameters comprise a magnitude of starting power and a magnitude
of a starting voltage.
23. A system as in claim 19, wherein said subsequent RF energy
pulses are each varied in amplitude by a controlled amount from a
previous RF energy pulse.
24. A system as in claim 19, further comprising one of a plurality
of pulse parameter lookup tables that is readably coupled to said
controller, and wherein said controller, when determining said
initial set of pulse parameters, uses said impedance value to
readout said initial set of pulse parameters from said one of the
plurality of pulse parameter lookup tables.
25. A system as in claim 19, wherein said one of a plurality of
pulse parameter lookup tables is selected manually or
automatically, based on a choice of an electrosurgical tool or
instrument.
26. A system as in claim 19, wherein said controller is responsive
to a control input from an operator for modifying any one of said
pulse parameters.
27. A system as in claim 19, wherein said controller is responsive
to a determination that said electrical transient is absent for
terminating a generation of subsequent RF energy pulses.
Description
CLAIM OF PRIORITY FROM A COPENDING PATENT APPLICATION:
[0001] Priority is herewith claimed from co-pending U.S. patent
application having U.S. application Ser. No. 10/073,761 entitled
"Vessel Sealing System", by Robert Wham et al. The disclosure of
this patent application is incorporated by reference herein in its
entirety.
FIELD
[0002] This invention relates generally to medical instruments and,
in particular, to generators that provide radio frequency (RF)
energy useful in sealing tissue and vessels during electrosurgical
and other procedures.
BACKGROUND
[0003] Electrosurgical generators are employed by surgeons to cut
and coagulate the tissue of a patient. High frequency electrical
power, which may be also referred to as radio frequency (RF) power
or energy, is produced by the electrosurgical generator and applied
to the tissue by an electrosurgical tool. Both monopolar and
bipolar configurations are commonly used during electrosurgical
procedures.
[0004] Electrosurgical techniques can be used to seal small
diameter blood vessels and vascular bundles. Another application of
electrosurgical techniques is in tissue welding, wherein two layers
of tissue are grasped and clamped together by a suitable
electrosurgical tool while the electrosurgical RF energy is
applied. The two layers of tissue are then "welded" together.
[0005] At this point it is significant to note that the process of
coagulating small vessels is fundamentally different than vessel
sealing or tissue welding. For the purposes herein the term
coagulation can be defined as a process of desiccating tissue
wherein the tissue cells are ruptured and dried. Vessel sealing or
tissue welding can both be defined as desiccating tissue by the
process of liquefying the collagen in the tissue so that it
crosslinks and reforms into a fused mass. Thus, the coagulation of
small vessels if generally sufficient to close them, however,
larger vessels normally need to be sealed to assure permanent
closure.
[0006] However, and as employed herein, the term "electrosurgical
desiccation" is intended to encompass any tissue desiccation
procedure, including electrosurgical coagulation, desiccation,
vessel sealing, and tissue welding.
[0007] One of the problems that can arise from electrosurgical
desiccation is undesirable tissue damage due to thermal effects,
wherein otherwise healthy tissue surrounding the tissue to which
the electrosurgical energy is being applied is thermally damaged by
an effect known in the art as "thermal spread". During the
occurrence of thermal spread excess heat from the operative site
can be directly conducted to the adjacent tissue, and/or the
release of steam from the tissue being treated at the operative
site can result in damage to the surrounding tissue.
[0008] It can be appreciated that it would be desirable to provide
an electrosurgical generator that limited the possibility of the
occurrence of thermal spread.
[0009] Another problem that can arise with conventional
electrosurgical techniques is a buildup of eschar on the
electrosurgical tool or instrument. Eschar is a deposit that forms
on working surface(s) of the tool, and results from tissue that is
electrosurgically desiccated and then charred. One result of the
buildup of eschar is a reduction in the effectiveness of the
surgical tool. The buildup of eschar on the electrosurgical tool
can be reduced if less heat is developed at the operative site.
[0010] It has been well established that a measurement of the
electrical impedance of tissue provides an indication of the state
of desiccation of the tissue, and this observation has been
utilized in some electrosurgical generators to automatically
terminate the generation of electrosurgical power based on a
measurement of tissue impedance.
[0011] At least two techniques for determining an optimal amount of
desiccation are known by those skilled in this art. One technique
sets a threshold impedance, and terminates electrosurgical power
when the measured tissue impedance crosses the threshold. A second
technique terminates the generation of electrosurgical power based
on dynamic variations in the tissue impedance.
[0012] A discussion of the dynamic variations of tissue impedance
can be found in a publication entitled "Automatically Controlled
Bipolar Electrocoagulation", Neurosurgical Review, 7:2-3, pp.
187-190, 1984, by Vallfors and Bergdahl. FIG. 2 of this publication
depicts the impedance as a function of time during the heating of a
tissue, and the authors reported that the impedance value of tissue
was observed to be near to a minimum value at the moment of
coagulation. Based on this observation, the authors suggest a
micro-computer technique for monitoring the minimum impedance and
subsequently terminating the output power to avoid charring the
tissue.
[0013] Another publication by the same authors, "Studies on
Coagulation and the Development of an Automatic Computerized
Bipolar Coagulator", Journal of Neurosurgery, 75:1, pp. 148-151,
July 1991, discusses the impedance behavior of tissue and its
application to electrosurgical vessel sealing, and reports that the
impedance has a minimum value at the moment of coagulation.
[0014] The following U.S. patents are also of interest in this
area. U.S. Pat. No. 5,540,684, Hassler, Jr. addresses the problem
associated with turning off the RF energy output automatically
after the tissue impedance has fallen from a predetermined maximum,
subsequently risen from a predetermined minimum and then reached a
particular threshold. A storage device records maximum and minimum
impedance values, and a circuit determines the threshold. U.S. Pat.
No. 5,472,443, Cordis et al., discusses a variation of tissue
impedance with temperature, wherein the impedance is shown to fall,
and then to rise, as the temperature is increased. FIG. 2 of this
patent shows a relatively lower temperature Region A where salts
contained in body fluids are believed to dissociate, thereby
decreasing the electrical impedance. A relatively next higher
temperature Region B is where the water in the tissue boils away,
causing the impedance to rise. The next relatively higher
temperature Region C is where the tissue becomes charred, which
results in a slight lowering of the electrical impedance. U.S. Pat.
No. 4,191,188, Belt et al., discloses the use of two timers whose
duty cycles are simultaneously and proportionately adjusted so that
high frequency signal bursts are constantly centered about the peak
power point, regardless of duty cycle variations.
[0015] Also of interest is U.S. Pat. No. 5,827,271, Buysse et al.,
"Energy Delivery System for Vessel Sealing", which employs a
surgical tool capable of grasping a tissue and applying an
appropriate amount of closure force to the tissue, and for then
conducting electrosurgical energy to the tissue concurrently with
the application of the closure force. FIG. 2 of this patent, shown
herein as FIG. 1 for depicting the prior art, illustrates a set of
power curves which represent the electrosurgical power delivered to
the tissue as a function of the tissue impedance. At low
impedances, the electrosurgical power is increased by rapidly
increasing the output current. The increase in electrosurgical
power is terminated when a first impedance breakpoint, labeled as
1, is reached (e.g. <20 ohms). Next, the electrosurgical power
is held approximately constant until proteins in the vessels and
other tissues have melted. The impedance at which this segment ends
varies in accordance with the magnitude of the RMS power. For
example, where the maximum RMS power is approximately 125 Watts,
the segment (B) ends at about 128 ohms. When a lower power is used
(e.g., 75 Watts), the segment (C) may end at an impedance value of
256 ohms. Next, the output power is lowered to less than one half
the maximum value, and the lower power delivery is terminated when
a second impedance breakpoint is reached (2.048.times.10.sup.3
ohms). Alternatives to using the impedance for determining the
second breakpoint are the use of I--V phase angle, or the magnitude
of the output current.
[0016] Based on the foregoing it should be evident that
electrosurgery requires the controlled application of RF energy to
an operative tissue site. To achieve successful clinical results
during surgery, the electrosurgical generator should produce a
controlled output RF signal having an amplitude and wave shape that
is applied to the tissue within predetermined operating levels.
However, problems can arise during electrosurgery when rapid
desiccation of tissue occurs resulting in excess RF levels being
applied to the tissue. These excess levels produce less than
desirable tissue effects, which can increase thermal spread, or can
cause tissue charring and may shred and disintegrate tissue. It
would be desirable to provide a system with more controlled output
to improve vessel sealing and reduce damage to surrounding tissue.
The factors that affect vessel sealing include the surgical
instrument utilized, as well as the generator for applying RF
energy to the instrument jaws. It has been recognized that the gap
between the instrument jaws and the pressure of the jaws against
the tissue affect tissue sealing because of their impact on current
flow. For example, insufficient pressure or an excessive gap will
not supply sufficient energy to the tissue and could result in an
inadequate seal.
[0017] However, it has also been recognized that the application of
RF energy also affects the seal. For example, pulsing of RF energy
will improve the seal. This is because the tissue loses moisture as
it desiccates and by stopping or significantly lowering the output
the generator between pulses, this allows some moisture to return
to the tissue for the application of next RF pulse. It has also
been recognized by the inventors that varying each pulse dependent
on certain parameters is also advantageous in providing an improved
seal. Thus, it would be advantageous to provide a vessel sealing
system which better controls RF energy and which can be varied at
the outset of the procedure to accommodate different tissue
structures, and which can further be varied during the procedure
itself to accommodate changes in the tissue as it desiccates.
[0018] An accommodation for overvoltage clamping is also desirable.
In this regard, conventional overvoltage techniques use a means of
clamping or clipping the excess overvoltage using avalanche devices
such as diodes, zener diodes and transorbs so as to limit the
operating levels. In these techniques the excess energy, as well as
the forward conduction energy, is absorbed by the protection device
and inefficiently dissipated in the form of heat. More advanced
prior art techniques actively clamp only the excess energy using a
predetermined comparator reference value, but still absorb and
dissipate the excess energy in the form of heat.
[0019] U.S. Pat. No. 5,594,636 discloses a system for AC to AC
power conversion using switched commutation. This system addresses
overvoltage conditions which occur during switched commutation by
incorporating an active output voltage sensing and clamping using
an active clamp voltage regulator which energizes to limit the
output. The active clamp switches in a resistive load to dissipate
the excess energy caused by the overvoltage condition.
[0020] Other patents in this area include U.S. Pat. No. 5,500,616,
which discloses an overvoltage clamp circuit, and U.S. Pat. No.
5,596,466, which discloses an isolated half-bridge power module.
Both of these patents identify output overvoltage limiting for all
power devices, and overvoltage limit protection is provided for
power devices by using proportionately scaled zeners to monitor and
track the output off voltage of each device to prevent power device
failure. The zener device is circuit configured such that it
provides feedback to the gate of the power device, When zener
avalanche occurs the power device partially turns on, absorbing the
excess overvoltage energy in conjunction with the connective
load.
[0021] Reference can also be had to U.S. Pat. No. 4,646,222 for
disclosing an inverter incorporating overvoltage clamping.
Overvoltage clamping is provided by using diode clamping devices
referenced to DC power sources. The DC power sources provide a
predetermined reference voltage to clamp the overvoltage condition,
absorbing the excess energy through clamp diodes which dissipate
the excess voltage in the form of heat.
[0022] It would be advantageous as to provide an electrosurgical
generator having improved overvoltage limit and transient energy
suppression.
SUMMARY
[0023] The foregoing and other problems are overcome by methods and
apparatus in accordance with embodiments disclosed herein.
[0024] An electrosurgical generator includes a controlling data
processor that executes software algorithms providing a number of
new and useful features. These features preferably include the
generation of an initial pulse, that is a low power pulse of RF
energy that is used to sense at least one electrical characteristic
of the tissue prior to starting an electrosurgical desiccation
cycle, such as a tissue sealing cycle. The sensed electrical
characteristic is then used as an input into the determination of
initial sealing parameters, thereby making the sealing procedure
adaptive to the characteristics of the tissue to be sealed. Another
feature preferably provided measures the time required for the
tissue to begin desiccating, preferably by observing an electrical
transient at the beginning of an RF energy pulse, to determine
and/or modify further seal parameters. Another preferable feature
performs a tissue temperature control function by adjusting the
duty cycle of the RF energy pulses applied to the tissue, thereby
avoiding the problems that can result from excessive tissue
heating. A further preferable feature controllably decreases the RF
pulse voltage with each pulse of RF energy so that as the tissue
desiccates and shrinks (thereby reducing the spacing between the
surgical tool electrodes), arcing between the electrodes is
avoided, as is the tissue destruction that may result from
uncontrolled arcing. Preferably a Seal Intensity operator control
is provided that enables the operator to control the sealing of
tissue by varying parameters other than simply the RF power.
[0025] The system disclosed herein preferably further provides a
unique method for overvoltage limiting and transient energy
suppression. An electrosurgical system uses dynamic, real-time
automatic detuning of the RF energy delivered to the tissue of
interest. More specifically, this technique automatically limits
excess output RF voltages by dynamically changing the tuning in a
resonant source of RF electrosurgical energy, and by altering the
shape of the RF source signal used to develop the output RF signal.
The inventive technique limits the excess output transient RF
energy by a resonant detuning of the generator. This occurs in a
manner which does not clip or significantly distort the generated
RF output signal used in a clinical environment for electrosurgical
applications.
[0026] A method for electrosurgically sealing a tissue, in
accordance with this disclosure, preferably includes the steps of
(A) applying an initial pulse of RF energy to the tissue, the pulse
having characteristics selected so as not to appreciably heat the
tissue; (B) measuring a value of at least one electrical
characteristic of the tissue in response to the applied first
pulse; (C) in accordance with the measured at least one electrical
characteristic, determining an initial set of pulse parameters for
use during a first RF energy pulse that is applied to the tissue;
and (D) varying the pulse parameters of subsequent RF energy pulses
individually in accordance with at least one characteristic of an
electrical transient that occurs at the beginning of each
individual subsequent PF energy pulse. The method terminates the
generation of subsequent RF energy pulses based upon a reduction in
the output voltage or upon a determination that the electrical
transient is absent.
[0027] The at least one characteristic that controls the variation
of the pulse parameters is preferably a width of the electrical
transient that occurs at the beginning of each subsequent RF energy
pulse. The initial set of pulse parameters include a magnitude of a
starting power and a magnitude of a starting voltage, and the pulse
parameters that are varied include a pulse duty cycle and a pulse
amplitude. Preferably, the subsequent RF energy pulses are each
reduced in amplitude by a controlled amount from a previous RF
energy pulse, thereby compensating for a decrease in the spacing
between the surgical tool electrodes due to desiccation of the
tissue between the electrodes.
[0028] The step of determining an initial set of pulse parameters
preferably includes a step of using the measured value of at least
one electrical characteristic of the tissue to readout the initial
set of pulse parameters from an entry in a lookup table.
[0029] The step of determining an initial set of pulse parameters
may also preferably include a step of reading out the initial set
of pulse parameters from an entry in one of a plurality of lookup
tables, where the lookup table is selected either manually or
automatically, based on the electrosurgical instrument or tool that
is being used.
[0030] The method also preferably includes a step of modifying
predetermined ones of the pulse parameters in accordance with a
control input from an operator. The predetermined ones of the pulse
parameters that are modified include a pulse power, a pulse
starting voltage level, a pulse voltage decay scale factor, and a
pulse dwell time.
[0031] Preferably a circuit is coupled to the output of the
electrosurgical generator for protecting the output against an
overvoltage condition, and includes a suppressor that detunes a
tuned resonant circuit at the output for reducing a magnitude of a
voltage appearing at the output. In accordance with this aspect of
the disclosure, the circuit has a capacitance network in parallel
with an inductance that forms a portion of the output stage of the
generator. A voltage actuated switch, such as a transorb, couples
an additional capacitance across the network upon an occurrence of
an overvoltage condition, thereby detuning the resonant network and
reducing the magnitude of the voltage output.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] The above set forth and other features of the invention are
made more apparent in the ensuing Detailed Description when read in
conjunction with the attached Drawings, wherein;
[0033] FIG. 1A is a graph that plots output power versus tissue
impedance (Z) in ohms, in accordance with the operation of a prior
art electrosurgical generator;
[0034] FIG. 1B is a graph that plots output power versus impedance
in ohms, in accordance with the operation of an electrosurgical
generator that is an aspect of this disclosure;
[0035] FIG. 2 is a simplified block diagram of an electrosurgical
system that can be used to practice the teachings of this
disclosure;
[0036] FIG. 3 is a perspective view of one embodiment of a surgical
instrument having bipolar forceps that are suitable for practicing
this disclosure;
[0037] FIG. 4 is an enlarged, perspective view of a distal end of
the bipolar forceps shown in FIG. 3;
[0038] FIG. 5 is a perspective view of an embodiment of a surgical
instrument having forceps that are suitable for use in an
endoscopic surgical procedure utilizing the electrosurgical system
disclosed herein;
[0039] FIG. 6A is a simplified block diagram of a presently
preferred embodiment of the power control circuit of the
electrosurgical generator of FIG. 2;
[0040] FIG. 6B depicts the organization of a seal parameter lookup
table (LUT) shown in FIG. 6A;
[0041] FIG. 7 illustrates a presently preferred electrosurgical
generator output waveform of RMS current vs. time for implementing
at least the first pulse of the pulsed operation mode that is an
aspect of this disclosure;
[0042] FIG. 8 depicts a full set of electrosurgical RF pulses in
accordance with this disclosure, and illustrates the voltage,
current and power characteristics of the pulses, as well as the
response of the tissue impedance to the applied RF pulses;
[0043] FIG. 9A illustrates a Seal Intensity control that forms a
part of this disclosure, while FIG. 9B shows a presently preferred
variation in certain parameters from the seal parameter LUT based
on different Seal Intensity settings;
[0044] FIG. 10 is a simplified block diagram of a circuit for
achieving an overvoltage limiting and transient energy suppression
energy function;
[0045] FIG. 11 is a waveform diagram illustrating the effect of the
operation of the circuit in FIG. 10;
[0046] FIG. 12 is a logic flow diagram that illustrates a method in
accordance with the system disclosed herein;
[0047] FIG. 13 is a more detailed logic flow diagram that
illustrates a method in accordance with the system disclosed
herein;
[0048] FIG. 14 is a chart illustrating a fixed number of pulses
determined from the measured impedance and the RMS current pulse
width;
[0049] FIG. 15 illustrates a Precise Seal Intensity control that
forms a part of this disclosure; and
[0050] FIG. 16 is a logic flow diagram that illustrates another
method in accordance with the system disclosed herein.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0051] An electrosurgical system 1, which can be used to practice
this invention, is shown in FIG. 2. The system 1 can be used for
sealing vessels 3 and other tissues of a patient, including ducts,
veins, arteries and vascular tissue. The system 1 includes an
electro-surgical generator 2 and a surgical tool, also referred to
herein as a surgical instrument 4. The surgical instrument 4 is
illustrated by way of example, and as will become apparent from the
discussion below, other instruments can be utilized. The
electrosurgical generator 2, which is of most interest to the
teachings herein, includes several interconnected sub-units,
including an RF drive circuit 2A, a power control circuit 2B, a
variable D.C. power supply 2C and an output amplifier 2D. The
surgical instrument 4 is electrically connected to the
electrosurgical generator 2 using a plug 5 for receiving controlled
electrosurgical power therefrom. The surgical instrument 4 has some
type of end effector member 6, such as a forceps or hemostat,
capable of grasping and holding the vessels and tissues of the
patient. The member 6, also referred to simply as end effector 6,
is assumed, in this embodiment, to be capable of applying and
maintaining a relatively constant level of pressure on the vessel
3.
[0052] The member 6 is provided in the form of bipolar
electrosurgical forceps using two generally opposing electrodes
disposed on inner opposing surfaces of the member 6, and which are
both electrically coupled to the output of the electrosurgical
generator 2. During use, different electric potentials are applied
to each electrode. In that tissue is an electrical conductor, when
the forceps are utilized to clamp or grasp the vessel 3
therebetween, the electrical energy output from the electrosurgical
generator 2 is transferred through the intervening tissue. Both
open surgical procedures and endoscopic surgical procedures can be
performed with suitably adapted surgical instruments 4. It should
also be noted that the member 6 could be monopolar forceps that
utilize one active electrode, with the other (return) electrode or
pad being attached externally to the patient, or a combination of
bipolar and monopolar forceps.
[0053] By way of further explanation, FIG. 3 is a perspective view
of one embodiment of the surgical instrument 4 having a bipolar end
effector implemented as forceps 6A while FIG. 4 is an enlarged,
perspective view of a distal end of the bipolar forceps 6A shown in
FIG. 3.
[0054] Referring now to FIGS. 3 and 4, a bipolar surgical
instrument 4 for use with open surgical procedures includes a
mechanical forceps 20 and an electrode assembly 21. In the drawings
and in the description which follows, the term "proximal", as is
traditional, refers to the end of the instrument 4 which is closer
to the user, while the term "distal" refers to the end which is
further from the user.
[0055] Mechanical forceps 20 includes first and second members 9
and 11 which each have an elongated shaft 12 and 14, respectively.
Shafts 12 and 14 each include a proximal end and a distal end. Each
proximal end of each shaft portion 12, 14 includes a handle member
16 and 18 attached thereto to allow a user to effect movement of
the two shaft portions 12 and 14 relative to one another. Extending
from the distal end of each shaft portion 12 and 14 are end
effectors 22 and 24, respectively. The end effectors 22 and 24 are
movable relative to one another in response to movement of handle
members 16 and 18. These end effectors members 6A can be referred
to collectively as bipolar forceps.
[0056] Preferably, shaft portions 12 and 14 are affixed to one
another at a point proximate the end effectors 22 and 24 about a
pivot 25. As such, movement of the handles 16 and 18 imparts
movement of the end effectors 22 and 24 from an open position,
wherein the end effectors 22 and 24 are disposed in spaced relation
relative to one another, to a clamping or closed position, wherein
the end effectors 22 and 24 cooperate to grasp the tubular vessel 3
therebetween. Either one or both of the end effectors 22, 24 can be
movable.
[0057] As is best seen in FIG. 4, end effector 24 includes an upper
or first jaw member 44 which has an inner facing surface and a
plurality of mechanical interfaces disposed thereon which are
dimensioned to releasable engage a portion of an electrode assembly
21, which may be disposable. Preferably, the mechanical interfaces
include sockets 41 which are disposed at least partially through
the inner facing surface of jaw member 44 and which are dimensioned
to receive a complimentary detent attached to an upper electrode
21A of the disposable electrode assembly 21. The upper electrode
21A is disposed across from a corresponding lower electrode 21B.
The end effector 22 includes a second or lower jaw member 42 which
has an inner facing surface which opposes the inner facing surface
of the first jaw member 44.
[0058] Preferably, shaft members 12 and 14 of the mechanical
forceps 20 are designed to transmit a particular desired force to
the opposing inner facing surfaces of the jaw members 22 and 24
when clamped. In particular, since the shaft members 12 and 14
effectively act together in a spring-like manner (i.e., bending
that behaves like a spring), the length, width, height and
deflection of the shaft members 12 and 14 directly impacts the
overall transmitted force imposed on opposing jaw members 42 and
44. Preferably, jaw members 22 and 24 are more rigid than the shaft
members 12 and 14 and the strain energy stored in the shaft members
12 and 14 provides a constant closure force between the jaw members
42 and 44.
[0059] Each shaft member 12 and 14 also includes a ratchet portion
32 and 34. Preferably, each ratchet, e.g., 32, extends from the
proximal end of its respective shaft member 12 towards the other
ratchet 34 in a generally vertically aligned manner such that the
inner facing surfaces of each ratchet 32 and 34 abut one another
when the end effectors 22 and 24 are moved from the open position
to the closed position. Each ratchet 32 and 34 includes a plurality
of flanges which project from the inner facing surface of each
ratchet 32 and 34 such that the ratchets 32 and 34 can interlock in
at least one position. In the embodiment shown in FIG. 3, the
ratchets 32 and 34 interlock at several different positions.
Preferably, each ratchet position holds a specific, i.e., constant,
strain energy in the shaft members 12 and 14 which, in turn,
transmits a specific force to the end effectors 22 and 24 and,
thus, to the electrodes 21A and 21B. Also, preferably a stop is
provided on one or both of the end effectors 22, 24 to maintain a
minimum gap between the jaws.
[0060] In some cases it may be preferable to include other
mechanisms to control and/or limit the movement of the jaw members
42 and 44 relative to one another. For example, a ratchet and pawl
system could be utilized to segment the movement of the two handles
into discrete units which, in turn, impart discrete movement to the
jaw members 42 and 44 relative to one another.
[0061] FIG. 5 is a perspective view of an embodiment of the
surgical instrument 4 having end effector members or forceps 6B
that are suitable for an endoscopic surgical procedure. The end
effector member 6B is depicted as sealing the tubular vessel 3
through a cannula assembly 130, 132.
[0062] The surgical instrument 4 for use with endoscopic surgical
procedures includes a drive rod assembly 50 which is coupled to a
handle assembly 54. The drive rod assembly 50 includes an elongated
hollow shaft portion 52 having a proximal end and a distal end. An
end effector assembly 6B is attached to the distal end of shaft 52
and includes a pair of opposing jaw members. Preferably, handle
assembly 54 is attached to the proximal end of shaft 52 and
includes an activator 56 for imparting movement of the forceps jaw
members of end effector member 6B from an open position, wherein
the jaw members are disposed in spaced relation relative to one
another, to a clamping or closed position, wherein the jaw members
cooperate to grasp tissue therebetween.
[0063] Activator 56 includes a movable handle 58 having an aperture
60 defined therein for receiving at least one of the operator's
fingers and a fixed handle 62 having an aperture 64 defined therein
for receiving an operator's thumb. Movable handle 58 is selectively
moveable from a first position relative to fixed handle 62 to a
second position in the fixed handle 62 to close the jaw members.
Preferably, fixed handle 62 includes a channel 66 which extends
proximally for receiving a ratchet 68 which is coupled to movable
handle 58. This structure allows for progressive closure of the end
effector assembly, as well as a locking engagement of the opposing
jaw members. In some cases it may be preferable to include other
mechanisms to control and/or limit the movement of handle 58
relative to handle 62 such as, e.g., hydraulic, semi-hydraulic
and/or gearing systems. As with instrument 4, a stop can also be
provided to maintain a minimum gap between the jaw members.
[0064] The handle 62 includes handle sections 62a and 62b, and is
generally hollow such that a cavity is formed therein for housing
various internal components. For example, the cavity can house a PC
board which controls the electrosurgical energy being transmitted
from the electrosurgical generator 2 to each jaw member, via
connector 5. More particularly, electrosurgical energy generated
from the electrosurgical generator 2 is transmitted to the handle
PC board by a cable 5A. The PC board converts the electrosurgical
energy from the generator into two different electrical potentials
which are transmitted to each jaw member by a separate terminal
clip. The handle 62 may also house circuitry that communicates with
the generator 2, for example, identifying characteristics of the
electrosurgical tool 4 for use by the electrosurgical generator 2,
where the electrosurgical generator 2 may select a particular seal
parameter lookup table based on those characteristics (as described
below).
[0065] Preferably, a lost motion mechanism is positioned between
each of the handle sections 62a and 62b for maintaining a
predetermined or maximum clamping force for sealing tissue between
the jaw members.
[0066] Having thus described two exemplary and non-limiting
embodiments of surgical instruments 4 that can be employed with the
electrosurgical generator 2, a description will now be provided of
various aspects of the inventive electrosurgical generator 2.
[0067] FIG. 6A is a block diagram that illustrates the power
control circuit 2B of FIG. 2 in greater detail. The power control
circuit 2B includes a suitably programmed data processor 70 that is
preferably implemented as one or more microcontroller devices. In a
most preferred embodiment there are two principal microcontrollers,
referred to as a main microcontroller 70A and a feedback
microcontroller 70B. These two microcontrollers are capable of
communicating using shared data that is stored and retrieved from a
shared read/write memory 72. A control program for the data
processor 70 is stored in a program memory 74, and includes
software routines and algorithms for controlling the overall
operation of the electrosurgical generator 2. In general, the
feedback microcontroller 70B has a digital output bus coupled to an
input of a digital to analog converter (DAC) block 76 which outputs
an analog signal. This is a system control voltage (SCV), which is
applied to the variable DC power supply 2C to control the magnitude
of the voltage and current of output RF pulses,
[0068] An analog to digital converter (ADC) block 78 receives
analog inputs and sources a digital input bus of the feedback
microcontroller 70B. Using the ADC block 78 the microcontroller 70B
is apprised of the value of the actual output voltage and the
actual output current, thereby closing the feedback loop with the
SCV signal. The values of the output voltage and current can be
used for determining tissue impedance and for the overall, general
control of the applied RF energy waveform. It should be noted that
at least the ADC block 78 can be an internal block of the feedback
microcontroller 70B, and need not be a separate, external
component. It should be further noted that the same analog signals
can be digitized and read into the master microcontroller 70A,
thereby providing redundancy. The master microcontroller 70A
controls the state (on/off) of the high voltage (e.g., 190V max)
power supply as a safety precaution, controls the front panel
display(s), such as a Seal Intensity display, described below and
shown in FIG. 9A, and also receives various input switch closures,
such as a Seal Intensity selected by an operator.
[0069] It is noted that in a preferred embodiment of the
electrosurgical generator 2 a third (waveform) microcontroller 70C
is employed to generate the desired 470 kHz sinusoidal waveform
that forms the basis of the RF pulses applied to the tissue to be
sealed, such as the vessel 3 (FIG. 2). The waveform microcontroller
70C is controlled by the feedback microcontroller 70B and is
programmed thereby.
[0070] An output signal line from the feedback microcontroller 70B
is coupled to a Reset input of the waveform microcontroller 70C to
essentially turn the waveform microcontroller 70C on and off to
provide the pulsed RF signal in accordance with an aspect of this
disclosure. This particular arrangement is, of course, not to be
viewed in a limiting sense upon the practice of this system, as
those skilled in the art may derive a number of methods and
circuits for generating the desired RF pulses in accordance with
the teachings found herein.
[0071] As an overview, the software algorithms executed by the data
processor 70 provide the following features. First, and referring
now also to the preferred waveform depicted in FIG. 7, a low power
initial pulse of RF energy is used to sense at least one electrical
characteristic of the tissue prior to starting the seal cycle.
Second, the sensed electrical characteristic of the tissue is used
as an input into the determination of the initial sealing
parameters, thereby making the sealing procedure adaptive to the
characteristics of the tissue to be sealed. Third, the technique
measures the time required for the tissue to begin desiccating,
preferably by observing an electrical transient, to determine
and/or modify further seal parameters. Fourth, the technique
performs a tissue temperature control function by adjusting the
duty cycle of RF pulses applied to the tissue, thereby avoiding
excessive tissue heating and the problems that arise from excessive
tissue heating. This is preferably accomplished by using at least
one calculated seal parameter related to the time required for the
tissue to begin desiccating. Fifth, the technique controllably
decreases the RF pulse voltage with each pulse of RF energy so that
as the tissue desiccates and shrinks (thereby reducing the spacing
between the surgical instrument electrodes), arcing between the
instrument electrodes (e.g. 21A and 21B of FIG. 4) is avoided, as
is the tissue destruction that may result from such uncontrolled
arcing. This is also preferably accomplished by using at least one
calculated seal parameter that is related to the time required for
the tissue to begin desiccating. Sixth, the above-mentioned Seal
Intensity front panel control (FIG. 9A) enables the operator to
control the sealing of tissue by varying parameters other than
simply the RF power. These various aspects of this disclosure are
now described in further detail.
[0072] Referring now also to the logic flow diagram of FIG. 13, the
impedance sensing feature is implemented at the beginning of the
seal cycle, wherein the electrosurgical generator 2 senses at least
one electrical characteristic of the tissue, for example,
impedance, I--V phase rotation, or the output current, by using a
short burst of RF energy (FIG. 13, Steps A and B). The electrical
characteristic of the tissue may be measured at any frequency or
power level, but preferably is performed at the same frequency as
the intended working frequency (e.g., 470 kHz). In a most preferred
case the short burst of RF energy (preferably less than about 200
millisecond, and more preferably about 100 millisecond) is a 470
kHz sine wave with approximately 5 W of power. The initial pulse RF
power is made low, and the pulse time is made as short as possible,
to enable an initial tissue electrical characteristic measurement
to be made without excessively heating the tissue.
[0073] In a most preferred embodiment the electrical characteristic
sensed is the tissue impedance which is employed to determine an
initial set of parameters that are input to the sealing algorithm,
and which are used to control the selection of sealing parameters,
including the starting power and voltage (FIG. 13, Step C).
Generally, if the sensed impedance is in the lower ranges, then the
initial power and starting voltage are made relatively lower, the
assumption being that the tissue will desiccate faster and require
less energy. If the sensed impedance is in the higher ranges, the
initial power and starting voltage are made relatively higher, the
assumption being that the tissue will desiccate slower and require
more energy.
[0074] In other embodiments at least one of any other tissue
electrical characteristic, for example, the voltage or current, can
be used to set the parameters. These initial parameters are
preferably modified in accordance with the setting of the Seal
Intensity control input (FIG. 13, Step D), as will be described in
further detail below.
[0075] Referring again to FIG. 13, Step C, the sensed impedance is
employed to determine which set of values are used from a seal
parameter lookup table (LUT) 80 (see FIGS. 6A and 6B). The seal
parameter look up table may one of a plurality that are stored in
the generator or accessible to the generator. Furthermore, the seal
parameter table may be selected, manually or automatically, based
on, for example, the electrosurgical tool or instrument being
employed. The specific values read from the seal parameter LUT 80
(FIG. 6B) are then adjusted based on the Seal Intensity front panel
setting 82 (FIG. 13, Step D), as is shown more clearly in FIGS. 9A
and 9B. In a preferred, but not limiting embodiment, the values
read from the seal parameter LUT 80 comprise the power, maximum
voltage, starting voltage, minimum voltage, voltage decay, voltage
ramp, maximum RF on time, maximum cool scale factor, pulse minimum,
pulse dwell time, pulse off time, and the desired pulse width. In a
preferred, but not limiting embodiment, the seal parameter values
adjusted by the Seal Intensity front panel setting 82 (FIGS. 9A and
9B) comprise the power, starting voltage, voltage decay, and pulse
dwell time.
[0076] FIG. 1B is a graph that plots output power versus impedance
in ohms for the disclosed electrosurgical generator. The plot
labeled "Intensity Bar 1" shows the electrosurgical generator power
output versus impedance when the "VLOW" setting 82A (FIG. 9A) of
the Seal Intensity front panel setting 82 is selected. The plot
labeled Intensity Bar 2 shows the power output of the
electrosurgical generator when the "LOW" setting 82B of the Seal
Intensity front panel setting 82 is selected. The plot labeled
Intensity Bars 3, 4, 5, shows the power output of the
electrosurgical generator when the "MED" 82C, "HIGH" 82D or VHIGH"
82E Seal Intensity front panel settings 82 are selected. The Seal
Intensity front panel settings 82 adjust the seal parameter values
as shown in FIG. 9B.
[0077] Discussing this aspect of the disclosure now in further
detail, and referring as well to FIGS. 7 and 8, the selected Seal
Parameter Table, adjusted by the Seal Intensity front panel
settings is then utilized by the RF energy generation system and an
initial RF sealing pulse is then started.
[0078] As each pulse of RF energy is applied to the tissue, the
current initially rises to a maximum (Pulse Peak) and then, as the
tissue desiccates and the impedance rises due to loss of moisture
in the tissue, the current falls. Reference in this regard can be
had to the circled areas designated as "A" in the I.sub.rms
waveform of FIG. 8. The actual width of the resulting electrical
transient, preferably a current transient "A", is an important
factor in determining what type and amount of tissue is between the
jaws (electrodes) of the surgical instrument 4 (measured from "Full
Power RF Start" to "Pulse Low and Stable".) The actual current
transient or pulse width is also employed to determine the changes
to, or the values of, the parameters of the pulse duty cycle
("Dwell Time") and to the reduction of the pulse voltage, as well
as other parameters. This parameter can also be used to determine
whether the tissue seal has been completed, or if the surgical
instrument 4 has shorted.
[0079] As an alternative to directly measuring the pulse width, the
rate of change of an electrical characteristic (for example
current, voltage, impedance, etc.) of the transient "A" (shown in
FIG. 7E) may be measured periodically (indicated by the reference
number 90 shown in FIG. 7B) over the time the transient occurs. The
rate of change of the electrical characteristic may be proportional
to the width .DELTA.t 95 of the transient "A", defined by the
relationship;
.DELTA.t.varies.de/dt
[0080] where de/dt is the change in the electrical characteristic
over time. This rate of change may then be used to provide an
indication of the width of the transient "A" in determining the
type and amount of tissue that is between the jaws (electrodes) of
the surgical instrument 4, as well as the subsequent pulse duty
cycle ("Dwell Time"), the amount of subsequent pulse voltage
reduction, as well as other parameters.
[0081] Referring to FIG. 13, Step E, a subsequent RF energy pulse
is applied to the tissue, and the pulse width of the leading edge
current transient is measured (FIG. 13, Step F), A determination is
made if the current transient is present, If it is, control passes
via connector "a" to Step H, otherwise control passes via connector
"b" to Step K.
[0082] Assuming that the current transient is present, and
referring to FIG. 13, Step H, if the current transient pulse is
wide, for example, approximately in the range of 500-1000 ms, then
one can assume the presence of a large amount of tissue, or tissue
that requires more RF energy to desiccate. Thus, the Dwell Time is
increased, and a small reduction is made in the amplitude of the
next RF pulse (see the Vrms waveform in FIG. 8, and FIG. 13, Step
I). If the current transient pulse is narrow, for example, about
250 ms or less (indicating that the tissue impedance rapidly rose),
then one can assume a small amount of tissue, or a tissue type that
requires little RF energy to desiccate is present. Other ranges of
current transient pulse widths can also be used. The relationship
between the current transient pulse width and the tissue
characteristics may be empirically derived. In this case the Dwell
Time can be made shorter, and a larger reduction in the amplitude
of the next RF pulse can be made as well (FIG. 13, Step J).
[0083] If a current pulse is not observed at FIG. 13, Step G, it
may be assumed that either the instrument 4 has shorted, the tissue
has not yet begun to desiccate, or that the tissue has been fully
desiccated and, thus, the seal cycle is complete.
[0084] The determination of which of the above has occurred is
preferably made by observing the tissue impedance at FIG. 13, Steps
K and M. If the impedance is less than a low threshold value
(THRESH.sub.L), then a shorted instrument 4 is assumed (FIG. 13,
Step L), while if the impedance is greater than a high threshold
value (THRESH.sub.H), then a complete tissue seal is assumed (FIG.
13, Step N).
[0085] If the tissue impedance is otherwise found to be between the
high and low threshold values, a determination is made as to
whether the Max RF On Time has been exceeded. If the Max RF On Time
has been exceeded, it is assumed that the seal cannot be
successfully completed for some reason and the sealing procedure is
terminated. If the Max RF On Time has not been exceeded then it is
assumed that the tissue has not yet received enough RF energy to
start desiccation, and the seal cycle continues (connector
"c").
[0086] After the actual pulse width measurement has been completed,
the Dwell Time is determined based on the actual pulse width and on
the Dwell Time field in the seal parameter LUT 80 (see FIG. 6B.)
The RF pulse is continued until the Dwell Time has elapsed,
effectively determining the total time that RF energy is delivered
for that pulse. The RF pulse is then reduced to a very low level
(effectively off) for an amount of time specified by the Pulse Off
field. This low level allows some moisture to return to the
tissue.
[0087] Based on the initial Desired Pulse Width field of the seal
parameter LUT 80 for the first pulse, or, for subsequent pulses,
the actual pulse width of the previous pulse, the desired voltage
limit is raised or lowered based on the Voltage Decay and Voltage
Ramp fields. The desired voltage limit is raised during the pulse
if the actual pulse width is greater than the Desired Pulse Width
field (or last actual pulse width), and is lowered if the actual
pulse width is less than the Desired Pulse Width field (or the last
actual pulse width).
[0088] When the Desired Voltage has been reduced to the Minimum
Voltage field, then the RF energy pulsing is terminated and the
electrosurgical generator 2 enters a cool-down period having a
duration that is set by the Maximum Cool SF field and the actual
pulse width of the first pulse.
[0089] Several of the foregoing and other terms are defined with
greater specificity as follows (see also FIG. 7).
[0090] The Actual Pulse width is the time from pulse start to pulse
low. The Pulse Peak is the point where the current reaches a
maximum value, and does not exceed this value for some
predetermined period of time (measured in milliseconds). The peak
value of the Pulse Peak can be reached until the Pulse Peak-X%
value is reached, which is the point where the current has
decreased to some predetermined determined percentage, X, of the
value of Pulse Peak. Pulse Low is the point where the current
reaches a low point, and does not go lower for another
predetermined period of time. The value of the Maximum RF On Time
or MAX Pulse Time is preferably preprogrammed to some value that
cannot be readily changed. The RF pulse is terminated automatically
if the Pulse Peak is reached but the Pulse Peak-X% value is not
obtained with the duration set by the Maximum RF On Time field of
the seal parameter LUT 80.
[0091] Referring to FIG. 6B, the seal parameter LUT 80 is employed
by the feedback microcontroller 70B in determining how to set the
various outputs that impact the RF output of the electrosurgical
generator 2. The seal parameter LUT 80 is partitioned into a
plurality of storage regions, each being associated with a
particular measured initial impedance. More particularly, the
Impedance Range defines a plurality of impedance breakpoints (in
ohms) which are employed to determine which set of variables are to
be used for a particular sealing cycle. The particular Impedance
Range that is selected is based on the above described Impedance
Sense State (FIG. 7) that is executed at the start of the seal
cycle. The individual data fields of the seal parameter LUT 80 are
defined as follows.
[0092] The actual values for the Impedance Ranges of Low, Med Low,
Med High, or High, are preferably contained in one of a plurality
of tables stored in the generator 2, or otherwise accessible to the
generator 2. A specific table may be selected automatically, for
example, based on signals received from the electrosurgical tool 4
being used, or by the operator indicating what electrosurgical tool
is in use.
[0093] Power is the RF power setting to be used (in Watts). Max
Voltage is the greatest value that the output voltage can achieve
(e.g., range 0--about 190V). Start Voltage is the greatest value
that the first pulse voltage can achieve (e.g., range 0--about
190V). Subsequent pulse voltage values are modified downwards from
this value. The Minimum Voltage is the voltage endpoint, and the
seal cycle can be assumed to be complete when the RF pulse voltage
has been reduced to this value. The Voltage Decay scale factor is
the rate (in volts) at which the desired voltage is lowered if the
current Actual Pulse Width is less than the Desired Pulse Width.
The Voltage Ramp scale factor is the rate at which the desired
voltage will be increased if the Actual Pulse Width is greater than
the Desired Pulse Width. The Maximum RF On Time is the maximum
amount of time (e.g., about one second) that the RF power can be
delivered, as described above. The Maximum Cool Down Time
determines the generator cool down time, also as described above.
Pulse Minimum establishes the minimum Desired Pulse Width value. It
can be noted that for each RF pulse, the Desired Pulse Width is
equal to the Actual Pulse Width from the previous pulse, or the
Desired Pulse field if the first pulse. The Dwell Time scale factor
was also discussed previously, and is the time (in milliseconds)
that the RF pulse is continued after the current drops to the Pulse
Low and Stable point (see FIG. 7). Pulse Off is the off time (in
milliseconds) between RF pulses. Desired Pulse Width is a targeted
pulse width and determines when the Desired Voltage (Vset) is
raised or lowered. If the Actual Pulse Width is less than the
Desired Pulse Width, then Vset is decreased, while if the Actual
Pulse Width is greater than the Desired Pulse Width, then Vset is
increased. The Desired Pulse Width is a value for the first pulse,
when the Actual Pulse Width is used as the Desired Pulse Width for
each sequential pulse. In general, a new Desired Pulse Width cannot
be greater than a previous Desired Pulse Width, and cannot be less
than Pulse Minimum.
[0094] By applying the series of RF pulses to the tissue, the
surgical generator 2 effectively raises the tissue temperature to a
certain level, and then maintains the temperature relatively
constant. If the RF pulse width is too long, then the tissue may be
excessively heated and may stick to the electrodes 21A, 21B of the
surgical instrument 4, and/or an explosive vaporization of tissue
fluid may damage the tissue, such as the vessel 3. If the RF pulse
width is too narrow, then the tissue will not reach a temperature
that is high enough to properly seal. As such, it can be
appreciated that a proper balance of duty cycle to tissue type is
important.
[0095] During the pulse off cycle that is made possible in
accordance with the teachings herein, the tissue relaxes, thereby
allowing the steam to exit without tissue destruction. The tissue
responds by rehydrating, which in turn lowers the tissue impedance.
The lower impedance allows the delivery of more current in the next
pulse. This type of pulsed operation thus tends to regulate the
tissue temperature so that the temperature does not rise to an
undesirable level, while still performing the desired
electrosurgical procedure, and may also allow more energy to be
delivered, and thus achieving better desiccation.
[0096] As each RF pulse is delivered to the tissue, the tissue
desiccates and shrinks due to pressure being applied by the jaws of
the surgical instrument 4. The inventors have realized that if the
voltage applied to the tissue is not reduced, then as the spacing
between the jaws of the surgical instrument 4 is gradually reduced
due to shrinking of the tissue, an undesirable arcing can develop
which may vaporize the tissue, resulting in bleeding.
[0097] As is made evident in the V.sub.rms trace of FIG. 8, and as
was described above, the voltage of each successive RF pulse can be
controllably decreased, thereby compensating for the
desiccation-induced narrowing of the gap between the surgical
instrument electrodes 21A and 21B. That is, the difference in
electric potential between the electrodes is decreased as the gap
between the electrodes decreases, thereby avoiding arcing.
[0098] As was noted previously, the Seal Intensity front panel
adjustment is not a simple RF power control. The adjustment of the
seal intensity is accomplished by adjusting the power of the
electrosurgical generator 2, as well as the generator voltage, the
duty cycle of the RF pulses, the length of time of the seal cycle
(e.g., number of RF pulses), and the rate of voltage reduction for
successive RF pulses. FIG. 9B illustrates an exemplary set of
parameters (Power, Start Voltage, Voltage Decay and Dwell Time),
and how they modify the contents of the seal parameter LUT 80
depending on the setting of the Seal Intensity control 82 shown in
FIG. 9A. Generally, higher settings of the Seal Intensity control
82 increase the seal time and the energy delivered while lower
settings decrease the seal time and the energy delivered.
[0099] It is instructive to note that for the Medium, High and Very
High Seal Intensity settings the RF Power remains unchanged, while
variations are made instead in the Start Voltage, Voltage Decay and
Dwell Time parameters.
[0100] Based on the foregoing it can be appreciated that an aspect
of this disclosure is a method for electrosurgically sealing a
tissue. Referring to FIG. 12, the method includes steps of: (A)
applying an initial pulse of RF energy to the tissue, the pulse
having characteristics selected so as not to appreciably heat the
tissue; (B) measuring at least one electrical characteristic of the
tissue in response to the applied pulse; (C) in accordance with the
measured electrical characteristic, determining an initial set of
pulse parameters for use during a first RF energy pulse that is
applied to the tissue; and (D) varying the pulse parameters of
individual ones of subsequent RF energy pulses in accordance with
at least one characteristic of an electric current transient that
occurs at the beginning of each individual one (pulses) of the
subsequent RF energy pulses. The method can terminate the
generation of subsequent RF energy pulses upon a determination that
the current transient is absent or that the voltage has been
reduced to a predefined level.
[0101] Reference is now made to FIGS. 10 and 11 for a description
of a novel over-voltage limit and transient energy suppression
aspect of the system disclosed herein.
[0102] A bi-directional transorb TS1 normally is non-operational.
As long as the operating RF output levels stay below the turn-on
threshold of TS1, electrosurgical energy is provided at a
controlled rate of tissue desiccation. However, in the event that
rapid tissue desiccation occurs, or that arcing is present in the
surgical tissue field, the RF output may exhibit operating voltage
levels in excess of the normal RF levels used to achieve the
controlled rate of tissue desiccation. If the excess voltage
present is left unrestrained, the tissue 3 may begin to exhibit
undesirable clinical effects contrary to the desired clinical
outcome. The TS1 is a strategic threshold that is set to turn on
above normal operating levels, but below and just prior to the RF
output reaching an excess voltage level where undesirable tissue
effects begin to occur. The voltage applied across TS1 is
proportionately scaled to follow the RF output voltage delivered to
the tissue 3. The transorb TS1 is selected such that its turn on
response is faster than the generator source RF signal. This allows
the transorb TS1 to automatically track and respond quickly in the
first cycle of an excess RF output overvoltage condition.
[0103] Note should be made in FIG. 10 of the capacitor components
or network C2, C3, and C4 that parallel the magnetic drive network
(MDN1) which has an inductive characteristic and is contained
within the electrosurgical generator 2. The combination of the
inductive MDN1 and the capacitive networks forms a resonant tuned
network which yields the waveshape configuration of the RF source
signal shown in FIG. 11.
[0104] A turn on of transorb device TS1, which functions as a
voltage controlled switch, instantaneously connects the serial
capacitance C1 across the capacitor network C2, C3, and C4. An
immediate change then appears in the tuning of the resonant network
mentioned above, which then instantaneously alters the waveshape of
the RF source signal shown in FIG. 11. The time base T1 of the
nominally half-sine signal shown increases incrementally in width
out to time T2, which automatically lowers the peak voltage of the
RF output signal. The peak voltage decreases because the
Voltage-Time product of the signal shown in FIG. 11 is constant for
a given operating quiescence. The concept of a Voltage-Time product
is well known to those skilled in the art, and is not further
discussed herein.
[0105] As the peak voltage decreases, the excess overvoltage is
automatically limited and is restricted to operating levels below
that which cause negative clinical effects. Once the excess RF
output voltage level falls below the transorb threshold, the TS1
device turns off and the electrosurgical generator 2 returns to a
controlled rate of tissue desiccation.
[0106] In the event that arcing is present in the surgical tissue
field, undesirable excess transient RF energy may exist and may be
reflected in the RF output of the electrosurgical generator 2. This
in turn may generate a corresponding excess RF output voltage that
creates sufficient transient overvoltage to turn on the transorb
TS1. In this condition the cycle repeats as described above, where
TS1 turns on, alters the resonant tuned network comprised of the
magnetic and capacitive components, and thus also alters the RF
source signal waveshape. This automatically reduces the excess
overvoltage.
[0107] In accordance with this aspect of the disclosure, the excess
RF transient energy is suppressed and the overvoltage is limited by
the dynamic, real-time automatic detuning of the RF energy
delivered to the tissue being treated.
[0108] It should be noted that the embodiment of FIGS. 10 and 11
can be used to improve the operation of conventional
electrosurgical generators, as well as with the novel pulsed output
electrosurgical generator 2 that was described previously.
[0109] In an additional embodiment the measured electrical
characteristic of the tissue, preferably the impedance (Z.sub.i),
and the RMS current pulse width (P.sub.w) may be used to determine
a fixed voltage reduction factor (V.sub.dec) to be used for
subsequent pulses, and to determine a fixed number of pulses
(P.sub.#) to be delivered for the sealing procedure. The
relationship among the voltage reduction factor, the measured
impedance and the RMS current pulse width may be defined as
V.sub.dec=F(Z.sub.I, P.sub.w), and the relationship among the
number of pulses, the measured impedance and the RMS current pulse
width may be defined as P.sub.#=F'(Z.sub.I, P.sub.w) In FIG. 14 a
fixed number of pulses, P.sub.#, 100 determined from the measured
impedance and the RMS current pulse width are shown. Each
subsequent pulse is reduced by the fixed voltage reduction factor
(V.sub.dec) 110, also determined from the measured impedance and
the RMS current pulse width.
[0110] The process then loops back to step A'. One of the plurality
of lookup tables is selected manually or automatically, based on a
choice of an electrosurgical tool or instrument.
[0111] While the system has been particularly shown and described
with respect to preferred embodiments thereof, it will be
understood by those skilled in the art that changes in form and
details may be made therein without departing from its scope and
spirit.
[0112] In a further additional embodiment, tissue sealing is
accomplished by the electrosurgical system described above by
continuously monitoring or sensing the tissue impedance rate of
change. If the rate of change increases above a predetermined
limit, then RF pulsing is automatically terminated by controlling
the electrosurgical generator 2 accordingly and any previously
changed pulse parameters (e.g., power, voltage and current
increments) are reset to the original default values. In this
embodiment, the ending tissue impedance, i.e., the tissue impedance
at the end of each RF pulse, is also continuously monitored or
sensed. The ending tissue impedance is then used to determine the
pulse parameters for the subsequent RF pulse; to determine if the
seal cycle should end (based on the ending impedance of the last
few RF pulses which did not change by more than a predetermined
amount); and to determine the duty cycle of the subsequent RF
pulse.
[0113] Further, in this embodiment, RF power, current and/or
voltage levels of subsequent RF pulses can be modified on a
pulse-by-pulse basis depending on whether the tissue has responded
to the previously applied RF energy or pulse (i.e., if the tissue
impedance has begun to rise). For example, if the tissue has not
responded to a previously applied RF pulse, the RF power output,
current and/or voltage levels are increased for the subsequent RF
pulse.
[0114] Hence, since these RF pulse parameters can subsequently be
modified following the initial RF pulse, the initial set of RF
pulse parameters, i.e., a magnitude of a starting RF power level, a
magnitude of a starting voltage level, and a magnitude of a
starting current level, are selected accordingly such that the
first or initial RF pulse does not appreciably heat the tissue. One
or more of these starting levels are modified during subsequent RF
pulses if the tissue has not responded to the previously applied RF
pulse which includes the initial RF pulse.
[0115] The above functions are implemented by a seal intensity
algorithm represented as a set of programmable instructions
configured for being executed by at least one processing unit of a
vessel sealing system. The vessel sealing system includes a Precise
Seal Intensity control panel for manually adjusting the starting
voltage level, in a similar fashion as described above with
reference to FIGS. 9A and 9B.
[0116] As shown in FIG. 15, a preferred Precise Seal Intensity
control panel of the present inventive embodiment includes six
settings, i.e., "Off" 150A, "VLOW" 150B, "LOW" 150C, "MED" 150D,
"HIGH" 150E and "VHIGH" 150F. The Precise Seal Intensity front
panel settings 150 adjust the seal parameter values of the Precise
Seal Parameter Table as shown by Table I. The selected Precise Seal
Parameter Table, adjusted by the Precise Seal Intensity front panel
settings 150 is then utilized by an RF generation system, as
described above, and an initial RF sealing pulse is then
started.
1TABLE I Precise Seal Intensity Parameter Table. Setting Power
Start Voltage Off N/A N/A Very Low Reduce by 25% Reduce by 25% Low
Reduce by 12.5% Reduce by 12.5% Med No Change No Change High
Increase by 12.5% Increase by 12.5% Very High Increase by 25%
Increase by 25%
[0117] FIG. 16 is a logic flow diagram that illustrates a method in
accordance with the vessel sealing system. At step A', a RF pulse
is applied to tissue. At step B', the tissue impedance rate of
change is continuously monitored. At step C', a determination is
made whether the tissue impedance rate of change has passed a
predetermined limit. If yes, at step D', RF pulsing is terminated
and any previously changed pulse parameters are reset back to the
original defaults. If no, the process proceeds to step E'.
[0118] At step E', a determination is made as to whether the RF
pulse has ended. If no, the process loops back to step B'. If yes,
the process proceeds to step F'. At step F', the ending tissue
impedance is measured. At step G', the measured ending tissue
impedance is used for determining if the seal cycle should end
(based on the ending impedance of the last few RF pulses which did
not change by more than a predetermined amount). If yes, the
process terminates at step H'. If no, the process continues at step
I', where the ending tissue impedance is used for determining the
pulse parameters, i.e., the power, current and/or voltage levels,
and the duty cycle of the subsequent RF pulse from an entry in one
of a plurality of lookup tables.
* * * * *